Dissecting the cohesin mechanism of action by domain swapping and flexible loop deletion

Cohesin is an evolutionary conserved, four-subunit protein complex that shapes the 3-D landscape of chromatin in the cell nucleus. Its functions include ensuring the fidelity of chromosome segregation during mitosis and meiosis, preserving genome stability, and regulating gene expression. Mutations in cohesin subunits or its auxiliary factors are associated with developmental disorders and cancer. The Structural Maintenance of Chromosome (SMC) family proteins Smc1 and Smc3 are located at the heart of the cohesin complex. SMCs are elongated proteins composed of two globular domains called the hinge and the head, connected by a long anti-parallel coiled coil. Smc1 and Smc3 dimerize through their heads and hinges domains to form a ring. Do the hinges are simply a dimerization interface or alternatively, play an active role in cohesin function? To solve this enigma, we swapped the hinges of Smc1 and Smc3 and explored their ability to form functional cohesin. In addition, we deleted flexible loops located in the SMC heads to assess their importance to cohesion. We found that these loops are essential for cohesin activity and explored how these loops affect the cohesin mechanism of action. Altogether, our study provides new insight into the mechanism of one of the most sophisticated complexes that control chromatin structure with an impact on fundamental cellular processes and human health.